Solid state imager devices which include charge-coupled-devices (CCD) and complementary metal oxide semiconductor (CMOS), have commonly been used in photo-imaging applications.
Imager devices typically contain thousands of pixels in a pixel array on a single chip. Pixels convert light into an electrical signal that can then be stored and recalled by an electrical device such as, for example, a processor. The electrical signals that are stored may be recalled to produce an image on, for example, a computer screen or a printable media.
Exemplary CMOS imaging circuits, processing steps thereof, and detailed descriptions of the functions of various CMOS elements of an imaging circuit are described, for example, in U.S. Pat. Nos. 6,140,630, 6,376,868, 6,310,366, 6,326,652, 6,204,524, and 6,333,205, each of which is assigned to Micron Technology, Inc. The disclosures of each of the forgoing patents are hereby incorporated by reference in their entirety.
Solid state imager devices typically have an array of pixel cells containing photosensors, where each pixel cell produces a signal corresponding to the intensity of light impinging on that element when an image is focused on the array. These signals may then be used, for example, to display a corresponding image on a monitor or otherwise used to provide information about the optical image. The photosensors are typically photogates, phototransistors, photoconductors or photodiodes, where the conductivity of the photosensor corresponds to the intensity of light impinging on the photosensor. The magnitude of the signal produced by each pixel cell, therefore, is proportional to the amount of light impinging on the photosensor.
CMOS active pixel sensor (APS) solid state imaging devices are described, for example, in the foregoing patents. These imaging devices include an array of pixel cells, arranged in rows and columns, that convert light energy into electric signals. Each pixel includes a photodetector and one or more active transistors. The transistors typically provide amplification, read-out control and reset control, in addition to producing the electric signal output from the cell.
While CCD technology has a widespread use, CMOS imagers are being increasingly used as low cost imaging devices. A CMOS imager circuit includes a focal plane array of pixel cells, each one of the cells including a photoconversion device, for example, a photogate, photoconductor, phototransistor, or a photodiode for accumulating photo-generated charge in a portion of the substrate. A readout circuit is connected to each pixel cell and includes at least an output transistor, which receives photogenerated charges from a doped diffusion region and produces an output signal which is periodically read out through a pixel access transistor. The imager may optionally include a transistor for transferring charge from the photoconversion device to the diffusion region or the diffusion region may be directly connected to or be part of the photoconversion device. A transistor is also typically provided for resetting the diffusion region to a predetermined charge level before it receives the photoconverted charges.
In a CMOS imager, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to a floating diffusion region accompanied by charge amplification; (4) resetting the floating diffusion region to a known state; (5) selection of a pixel cell for readout; and (6) output and amplification of a signal representing the pixel cell charge. Photo-charge may be amplified when it moves from the initial charge accumulation region to the floating diffusion region. The charge at the floating diffusion region is typically converted to a pixel output voltage by a source follower output transistor.
Each pixel cell receives light focused through one or more micro-lenses. Micro-lenses on a CMOS imager help increase optical efficiency and reduce cross talk between pixel cells. A reduction of the size of the pixel cells allows for a greater number of pixel cells to be arranged in a specific pixel cell array, thereby increasing the resolution of the array.
To detect color, the spectral components of incident light must be separated and collected. An absorptive color filter array (CFA) on top of an imager chip is currently the dominant technology for color detection in a solid state image sensor, for example, a CCD or CMOS imager. In a typical CFA layout, a color filter for each individual photo sensor of the imager allows only a narrow spectral band (red, green, or blue) to pass, and absorbs the rest of photo energy. This greatly reduces the overall efficiency of the image sensor. Moreover, the color mosaic required for this technique increases the fabrication cost and complexity, and can lead to quality issues such as non-planarity and streaks.
There is needed, therefore, another method for color separation in solid state imagers compatible with imager fabrication processes.